Dimeritium is a special sort of metal which suppresses magical power. Vilgefortz of Roggeveen, Artaud Terranova and a few other Chapter members, were shackled with dimeritium after they was accused of treason by Philippa Eilhart and Dijkstra during the reunion of mages on Thanedd Island. It was stated that being shackled causes unpleasant side-effects for the mages.
"Dimeritium is a rare and precious metal with an interesting feature - it represses the transfer of magical energy. A bluish alloy of iron and dimeritium is used to produce handcuffs and necklaces. Those who wear them cannot cast spells or use magic in any way. There are known cases, however, of extraordinarily powerful sorcerers managing to overcome the shield generated by dimeritium."